The present invention relates to a process for the efficient removal of nitrogen oxide (NO.sub.X) from a waste gas using an adsorbing substance.
There are already several processes under development or in actual use for removing nitrogen oxide waste or exhaust gases such as from a boiler, electric generating facility or the like. These gases consist mainly of nitrogen oxide and contain small amounts of nitrogen dioxide. The processes are conveniently characterized as either a dry contact reduction process or a process of wet absorption removal. In both of these processes, due to low reactivity and relatively low concentrations of nitrogen oxide in the waste gas -- concentrations normally of about 100-900 ppm -- it has in the past been difficult to find an economical manner for nitrogen oxide removal. Concentrated efforts in this technology are being continued. Nitrogen oxide is an acknowledged air pollutant and considerable efforts have been undertaken to effect substantial removal of this source of pollution from fossel fuel combustion.
The known dry contact reduction process is a process which includes reacting the nitrogen oxide present in the waste gas with a reducing agent in a catalyst layer which consists of a metal or metal compound to selectively or nonselectively reduce the nitrogen oxide to a form of nitrogen which is pollution-free. Illustrative reducing agents include ammonia and hydrogen sulfide for selective reduction and hydrogen, methane and carbon monoxide for nonselective reduction. This process has distinct disadvantages including the requirement of a high operational temperature of above 300.degree. C for the reaction, reduction of the life of the catalyst due to the presence of sulfurous acid gases and requiring the additional treatment of an excess reducing agent in order to prevent pollution. In fact, in the event the reduction reaction is incomplete, there is a possibility that nitrous oxide is directly discharged. Also, an increased loss of pressure of the gas may occur due to blocking of the catalyst layer due to dust in the water gas. Many of these problems could be overcome if a reducing agent which would be operable at gas temperatures of about 50.degree.-70.degree. C after dust collection and desulfurization, that is, wet removal of sulfurous acid gas, of the waste gas can be found. Thus, the process of dry contact reduction would be considered an industrially effective process. But such catalyst has not yet been developed.
There are also various processes of wet absorption removal including converting nitrogen oxide (NO) into a nitric acid ion (NO.sub.3.sup.-) using an oxidizable solution and absorbing the ion into the solution, and a process for oxidizing NO to nitrogen dioxide, which is more reactive than NO, in a gaseous phase and thereafter absorbing NO.sub.2 in an appropriate absorbing liquid such as an alkaline solution to remove NO.sub.2.
As an example of the nitric acid ion conversion a process using a mixed solution of potassium permanganate and caustic soda is illustrative, but this process has drawbacks in that such an absorbing liquid is expensive and further a waste liquid containing nitric acid is difficult to properly treat for disposal. A process using hydrogen peroxide has many the same drawbacks in addition to the fact that an effective oxidizing amount of the hydrogen peroxide is relatively small due to self-decomposition.
With respect to oxidizing nitrogen oxide to nitrogen dioxide several oxidizing agents such as ozone, chlorous acid and a nitric acid solution have been proposed. However all of these procedures have their own disadvantages in that the cost of equipment and the cost of electrical power are very expensive (for the ozone process), the price of the oxidizing agent itself is fairly high and the resulting waste liquid must be treated (as with the chlorous acid process), and the reactivity is low when the concentration of nitric acid is 40.degree.-60% by weight, the reaction temperature is 40.degree.-70.degree. C and it is difficult to protect the operating apparatus from corrosion (as in the nitric acid solution process). Another proposal was using a solution of ferrous sulfate in absorbing nitrogen oxide in the form of a nitrogen monoxide complex but such a process has not proven to be practical either.